Answer: The only way to keep grass from sprouting up again is to either caulk the areas to eliminate sunlight or to brush some pre-emergent herbicide (such as casorone or preen) into the cracks on a regular basis. Start by using Round-Up to kill the existing weeds, then dig the dead roots out from between the slabs. Apply caulk to seal the gaps or sprinkle some pre-emergent into the gaps. That should do it!
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